Figure 4. (a) The hypothetical double aortic arch system proposed by Edwards, showing zones of the left arch labelled A–D (RC, right common carotid artery; RS, right subclavian artery; LC, left common carotid artery; LS, left subclavian artery; AA, ascending aorta; PA, pulmonary artery; rda, right ductus arteriosus; lda, left ductus arteriosus). (b) Atresia of left arch (LAA) along the zone D in hypothetical double aortic arch demonstrates subtype 4 of the double aortic arch with left atretic arch. Moreover, tapering (Tap) and aneurysm (An) are probably located, respectively, in the A and B zones, but the correct origin of the left ductus arteriosus (lda) from the left arch is uncertain (RC, right common carotid artery; RS, right subclavian artery; LC, left common carotid artery; LS, left subclavian artery; AA, ascending aorta; PA, pulmonary artery; rda, right ductus arteriosus; lda, left ductus arteriosus). Illustrations are modified from those by B Singh et al [8].